A comparative study of the lattice dynamics of alkali titanium hexahalides

Lattice dynamics calculations have been performed on a family of vacancy double perovskites of general formula A₂TiX₆ (A = Rb, Cs, X = F, Cl, Br, I) using density functional theory. This family has attracted interest as potential lead-free optoelectronic materials with tuneable band gaps. We find that most of these materials show stable lattice dynamics, but two, Rb₂TiF₆ and Rb₂TiI₆, show instabilities which would lead to a phase transition. In fact Rb₂TiF₆ normally crystallises in a phase with a different structure, which can be understood on the basis of calculations of the free energy.